One can see here that the Prussian family Pokrzywnicki (Bock) is a branch of the Mazovian Pokrzywnicki h. Jastrzembiec family (for the latter cf. Uruski, Seweryn hr.: Rodzina – Herbarz szlachty polskiej. Warszawa 1904-38; vol. 14 (1917) pp. 177-178).

The seals depicted here were all found as wax impressions on archived treaties or testaments of the persons mentioned with them. They show that the Prussian Pokrzywnicki changed their coat of arms in the 18th century to a form that has later been identified either as “Slepowron odmiana herb” (i.e. a variation of the coat of arms Slepowron) or as (a variation of) the coat of arms Lubicz.

My argument for Jastrzembiec to be the original coat of arms of this family clearly depends on Stanislaus Pokrzywnicki being right in using it. I would think that the probability of him knowing his family’s historically correct coat of arms is higher than that of the other individuals, for he lived earlier and in closer contact to the family’s Polish roots.

The correct Polish writing for the names of the coats of arms is:

• Jastrzembiec – Jastrzębiec
• Slepowron – Ślepowron

The wax impressions of seals I found show the following coats of arms:
 

Picture & identification by Polish name	Year	Seal used by
coat of arms Jastrzembiec	1718	Stanislaus de Pokrzywnicki (1665-1735), Royal Polish Lieutenant
„Slepowron odmiana herb“ –  variation of coat of arms Slepowron	1810	Johann Michael von Bock (1742-1812), Royal Prussian major general
„Slepowron odmiana herb“	1801	Adam von Bock (1752-1810), Royal Prussian major
„Slepowron odmiana herb“	1805   c.1800	Franz von Bock und Pokrzywnicki (1750-1830), Royal Prussian major   Two seals of the 18th century earlier in the possession of Paul Bock von Pokrzywnicki (1856-1928), probably from his grandfather Johann Andreas von Pokrzywnicki (1766-1831)

Here is how those persons are related: